This invention pertains to a buffer amplifier, more specifically to a DC-coupled high-impedance input amplifier for an oscilloscope or the like.
Buffer amplifiers find wide application throughout the electronic industry. For example, wideband analog test and measurement instruments such as oscilloscopes require high impedance, highly stable, DC-coupled wideband input amplifiers. Buffer amplifiers herein refer to such input stage amplifiers having a high input impedance and a low output impedance.
Buffer amplifiers for this purpose typically include a DC-coupled source follower input field-effect transistor (FET) stage followed by an emitter follower output stage. One example of such conventional buffer amplifiers is shown in FIG. 1. The gate of source follower FET 14 is DC coupled to input terminal 10 and also connected to a negative voltage source through normally off clamp diode 11. The drain of FET 14 is connected to a positive voltage source while the source thereof is connected through a parallel combination of resistor 20 and capacitor 22 to a constant current source including another FET 16 and source resistor 24. The drain of FET 16 is directly coupled to the base of emitter follower transistor 18, the emitter of which is connected through resistor 30 to a positive voltage source and also to output terminal 12. The collector of transistor 18 is returned to a negative voltage source. Resistor 15 is connected between input terminal 10 and ground to determine input resistance of the amplifier.
Under no-signal condition, when the gate voltage of FET 14 is zero volts, proper parameter selection allows both FETs 14 and 16 to operate on the same DC level. That is, the drain voltage of FET 16 and the gate voltage of FET 14 are essentially equal to each other and resistors 20 and 24 have substantially equal resistance. In this case, the gate-to-source voltages and drain currents of both FETs 14 and 16 are identical. This establishes essentially zero voltage on the base of transistor 18. FET 16 provides temperature compensation for FET 14. For this purpose, FETs 14 and 16 must be a matched pair of high cutoff frequency and are relatively expensive.
When the input gate voltage increases, the source current of FET 14 exceeds the constant drain current of FET 16, the resulting difference current flows into the base of transistor 18, thereby reducing the emitter current of transistor 18 to raise the emitter output voltage on output terminal 12. On the other hand, when the input voltage is negative, the source current of FET 14 is less than the constant drain current of FET 16, thereby the resulting difference current increases the base end emitter currents of transistor 18 to provide a negative output voltage at output terminal 12.
Drawbacks of the prior art circuit include difficulty of obtaining completely matched pair of FETs for wide range operation, high cost of the matched FET pair and voltage drift of transistor 18.
FIG. 2 shows another conventional buffer amplifier to solve some of the drawbacks of the FIG. 1 circuit. The biggest difference of FIG. 2 circuit over FIG. 1 circuit is the use of diode connected transistor 32 in the source circuit of FET 16. Transistor 32 with the base and collector thereof coupled to each other through resistor 34 preferably constitutes with emitter follower transistor 18' a matched bipolar transistor pair. FIG. 2 circuit operates much the same way as FIG. 1 circuit, but diode connected transistor 32 provides the temperature compensation of emitter follower transistor 18'. That is, when the ambient temperature rises to reduce the V.sub.BE of transistor 18', the V.sub.BE of transistor 32 also decreases, thereby increasing the drain current of FET 16. The base voltage of transistor 18' is reduced to keep the output voltage at output terminal 12 constant. The output voltage is, therefore, maintained substantially unchanged by the use of matched FET pair 14 and 16 and also matched bipolar transistor pair 18' and 32. Resistance of resistor 34 is chosen so that transistors 32 and 18' operate on a similar bias level to each other for better temperature compensation.
Basically, this invention features the use of a single source follower FET, a pair of bipolar transistors (not required to be a matched pair), and an operational amplifier. One bipolar transistor is connected in series with the source of the input stage source follower FET while the other is used as an output stage emitter follower transistor. A selected fraction of the input and output signals are compared with each other by the operational amplifier to provide low-frequency correction for the entire amplifier. The correction loop automatically maintains the output DC level constant.
The FIG. 2 circuit is more expensive than the FIG. 1 circuit because expensive matched bipolar transistor air 18' and 32 are required.
It is therefore a primary object of this invention to provide a buffer amplifier that is less expensive and less complicated in circuit arrangement.
It is another object of this invention to provide a wideband buffer amplifier with better electrical performance, expecially in DC stability than conventional amplifiers.
A feature of the buffer amplifier of the present invention is that it corrects for internally-generated low-frequency distortion.
Other objects, features, and advantages of the present invention will become apparent to those having skill in the art upon a reading of the following description when taken in conjunction with the accompanying drawings.